1. Field of the Invention
The invention relates to the field of metallurgical coatings and in particular to nanocrystalline thermal barrier coatings.
2. Description of the Prior Art
In order to increase the efficiency of gas turbine engines, the hot-section stationary components (mainly combustors, transition pieces, and vanes) are protected with thermal barrier coatings (TBCs). In addition to providing the thermal insulation to the nickel-based superalloy components, TBCs also provide protection against high temperature oxidation and hot corrosion attack. The conventional TBCs that are used in naval (diesel) engines, in military and commercial aircraft, and in land-based gas turbine engine components, consist of a duplex structure made up of a metallic MCrAlY (M stands for either Co, Ni and/or Fe) bond coat and Yttria partially stabilized zirconia (YPSZ) ceramic top coat.
Presently, however, the full potential of the YPSZ TBCs is yet to be realized due mainly to the cracking problem that occurs along or near the bond coat/top coat interface after a limited number of cycles of engine operation. This interfacial cracking, often leading to premature coating failure by debonding (spallation) of the top coat from the bond coat, has been amply demonstrated from microstructural evidence that was obtained from in-service degradation of deposited coatings as well as from laboratory experiments that have been conducted. The thin oxide layer that grows on top of the bond coat, at the bond coat/top coat interface, plays a critical role in the interface cracking. It is quite evident that this cracking problem negatively impacts the coating performance by reducing both the engine efficiency (because the engine operating temperature is kept below its optimum temperature) and the lifetime of the engine components. In turn, this greatly affects the reliability and the efficiency of the entire engine system.
In summary, coating failure at the interface has greatly limited the use of yttria-partially-stabilized zirconia TBCs both for military and civilian applications. The TBCs that are used to protect the hot-section components are deposited on nickel-based superalloy substrates by first depositing a MCrAlY bond coat and then depositing the YPSZ as the top coat. The typical thickness of the bond and top coats are 100/150 μm and 150/250 μm, respectively. Conventionally, two different thermal spray processes are used to deposit the TBC top coat: one is the electron beam physical vapor deposition (EB-PVD) process and the other is the atmospheric plasma spraying (APS) process; the metallic bond coat is typically deposited by vacuum plasma spray (VPS), by APS, and recently, by high-velocity oxy-fuel (HVOF).
The bond coat surface, onto which the YPSZ top coat is disposed, has a thin oxide layer that consists mostly of various oxides (NiO, Ni(Cr,Al)2O4, Cr2O3, Y2O3, Al2O3). The presence of this thin oxide layer plays an important role in the adhesion (bonding) between the metallic bond coat and the ceramic top coat. However, during engine operation, another oxide layer forms in addition to the native oxide. This second layer, also mostly alumina, is commonly referred to as the thermally grown oxide (TGO) and slowly grows during exposure to elevated temperatures. Interfacial oxides, in particular the TGO layer, play a pivotal role in the cracking process. It is believed that the growth of the TGO layer leads to the build up of stresses at the interface region between the TGO layer and top coat.
These stresses are mainly caused by the following two factors: (1) the thermo-mechanical mismatch between the metallic bond coat and the ceramic top coat; and, (2) a volume expansion at the interface region resulting from the formation of the TGO layer. Therefore, it is reasonable to assume that crack nucleation and its subsequent propagation occur when the interface stresses reach some critical value. That is to say, when the thickness of the TGO layer grows to some critical value.
Other mechanisms have been proposed to contribute to interface cracking. For example some researchers have suggested that a contributing factor might be the inhomogeneous distribution of oxide phases; others have hypothesized that the growth of the TGO can produce interfacial damage leading to microcracking and separation and that final failure is caused by the coalescence of these microcracks. Further, another proposed mechanism suggests that severe bond coat oxidation significantly lowers both the adhesion and the fracture energy between the top and the bond coats.
Although a great deal of papers and books have been published relating to thermal barrier coatings, there has yet to be any work on introducing a nanostructured, nano-composite bond coat layer below the top coat in the thermal barrier coatings.
One of the problems in applying a ceramic coating as found in a thermal barrier coating, relates to the mismatch in coefficient of thermal expansion between the two materials. This mismatch results in the erection of residual stress and a compromise in the bond strength at or near the interface. By incorporating a layer of single-composition or functionally graded nanostructure cermet composite layer, one can reduce the mismatch between the thermal barrier coatings. This could lead to a more thermal shock resistant top-and-bond-coat system.
The problem of thermally grown oxide growth and its negative effect on the top-and-bond-coat integrity has yet to be solved. The growth of thermally grown oxide has been found to be the major source of top-and-bond-coat failure for high temperature applications. As the oxide thermally grows, it exerts stress in the adjacent top coat; when this stress level exceeds the cohesive strength of the top coat material, failure in the form of spalling of the top coat occurs.